These type of structures are encountered in certain turbojet engines with counter-rotating propellers between the gas generator and the free outlet turbines. Radial arms frequently extend to this location dividing the annular stream into several arcs of a circle. Reference may be made here to U.S. Pat. No. 4,321,007 which shows the structure most closely resembling that of the invention. The stream is actually delimited by two internal and external circumferential walls resting respectively on internal and external collars disposed around each arm. However, the plates or sheets (known as "tiles" in this application) constituting the circumferential walls are welded together and to the arms, this having the drawback of resulting in providing an excessively rigid structure.